1. Field of the Invention
The present invention relates to a variable magnification optical system, and for example, relates to a variable magnification optical system (above all, a small-size and high-zoom-ratio zoom lens system) suitable for digital apparatuses capable of capturing an image of the subject by an image sensor, and an image forming device having the same.
2. Description of the Related Art
In recent years, with the spread of personal computers, digital still cameras with which images can be easily captured have been becoming widespread. With the spread of such digital still cameras, smaller-size digital still cameras have been required, and accordingly, taking lens systems have been required to be smaller in size. Moreover, since image magnification variation, in particular, optical magnification variation where image degradation is small is desired also for digital cameras for general consumers, various types of zoom lens systems have conventionally been proposed to meet the requirements for smaller size, higher performance and higher zoom ratio. For example, United States Patent Application Publication US 2003/0197947A1 discloses a zoom lens system in which the overall length of the optical system is reduced by omitting the infrared cut filter disposed immediately in front of the image sensor. This zoom lens system is a three-unit zoom lens system of negative, positive, positive configuration or a five-unit zoom lens system of positive, negative, positive, positive, positive configuration. Instead of the infrared cut filter, a wavelength selecting device is used in a predetermined zoom unit.
However, in this zoom lens system, since the wavelength selecting device is used in a lens unit that is movable in zooming, it is necessary to limit the light ray incident on the wavelength selecting device. That is, since the wavelength selecting device being used has an interference film on a lens surface, the wavelength selecting property varies according to the incident angle of the light ray, so that the incident angle is limited on the lens surfaces that are moved in zooming.